The electronic control device of low-voltage circuit breaker, such as electronic tripping unit, needs to be supplied with power, a built-in current transformer of a circuit breaker is generally utilized to obtain power from a primary main loop, electric power originates from a current flowing through a primary core-extending conductor, and an induced current in a secondary winding of the current transformer is supplied to electronic tripping unit for its operation.
At present, stronger functions of the electronic controller for low-voltage circuit breaker leads to larger power consumption of the electronic controller. Meanwhile, Perfection for protective function requires a lower protection starting point of the electronic controller. According to the national standard GB/T22710-2008 Electronic Controller for Low-Voltage Circuit Breaker in our country brought into effect on Oct. 1, 2009, a controller can work reliably and must implement the fundamental protective function when all phase currents in a main circuit are not less than 0.4 In (In is rated current) in the case of no auxiliary power source. According to the American national standard ANSI Std. C37.17-1997, however, a controller must complete the function of overload protection and ground fault protection in the case of no external auxiliary power source. As for the function of ground protection, the setting value of a protective current is 0.2 In to 1 In, that is, a transformer for supplying power to a controller has a secondary output so large that the controller works reliably and must implement the function of ground protection when a three-phase current of the primary main circuit is required to be minimally set to 0.2 In or single-phase 0.4 In. Therefore, the supply current transformer for an electronic controller has to be designed to satisfy the above operation conditions of controller. In other words, on the one hand, smaller primary current leads to wider range in which a controller can give its protection, and on the other hand, in case that the primary current is small enough as described above, the transformer is required to output a secondary current that is large enough.
Simultaneously, it is well known that a current transformer for power supply is typically a current transformer with cores. Input and output of such an core transformer are substantially linear within a particular range, and its secondary current varies based on variation of primary current. When a primary current reaches a normal starting current of the current transformer, the current transformer generates power sufficient to maintain reliable working of the controller, that is to say, the controller has a certain power consumption, and when the primary current increases once again, the current transformer for supplying power to an electronic controller generates power that significantly exceeds the power required for normal working of the electronic controller, in this case, excessive energy needs to be consumed in other ways, which undoubtedly requires an additional power consumption device. Hence, it is another major contradiction for such current transformers (typically known as self-regenerated power sources) to determine the way of acquiring a secondary current output, which is as steady as possible, instead of ceaseless increase, within an extremely wide primary current range from normal state to non-normal state after the secondary output of the current transformer meets the working demand of the controller. An ideal scheme for simultaneously solving the contradiction between the two aspects above has not been found yet for a long time. The difficulty falls not only upon the problem of structural scheme, but also upon the problem of optimization and matching for structural parameters.
Some structural design schemes for the magnetic shunt of current transformer has been worked out on the basis of electromagnetic principle, and these schemes featured by main magnetic circuit, auxiliary magnetic circuit and air gaps are approximately classified in two types below. One is as illustrated in U.S. Pat. No. 5,726,846A and CN 200110176191 in which a main magnetic circuit and an auxiliary magnetic circuit are not two independent magnetic circuits and air gaps are disposed in the auxiliary magnetic circuit, and what differs CN 200110176191 from U.S. Pat. No. 5,726,846A is that, the thickness of the air gaps in the former is variable, whereas the thickness of the air gaps in the latter is invariable. The other one is as illustrated in CN1637968.B in which a first magnetic circuit and a second magnetic circuit are two independent magnetic circuits each forming a closed loop, and the first magnetic circuit is operatively connected with the second magnetic circuit so that a certain proportion of main magnetic flux is absorbed by the second magnetic circuit before the main magnetic flux of the first magnetic circuit gets through the core of a secondary winding. The common defect in the prior arts above consists in an incapability of meeting two use demands simultaneously: 1. in the case that the primary current is 0.2 In to be small enough, the demand on normal start and work of the controller has to be met; and 2, in the case that the primary current is more than 1 In to be large enough (especially when the primary current is an overload current or a short circuit current), output of the secondary current can still be maintained under a stable state and normal work of the controller can be ensured. In the prior arts above, due to a plurality of factors like parameter matching, variation precision of variable air gaps, response speed and the like, the scheme featured by variable air gaps, though possibly advantageous for solving the above problems in terms of principle, is still a design under the state that is idealized, but fails to reach the ideal effect, and, instead, leads to new problems like complex structure, difficult assembly and debugging, etc.